The present invention relates to an automatic laser power control circuit for an optical disc system, and more particularly, to an automatic laser power control circuit in which a monitor diode for monitoring light power of a laser diode can be used regardless of its ground type.
An optical disc system such as a compact disc player (CDP) or a digital versatile disc player (DVDP) projects laser light on an information recording surface of a disc to read out information. A semiconductor laser diode which has been commonly used as a light source for laser light generally has a poor temperature characteristic. Therefore, the light power of the semiconductor laser diode decreases when temperature rises, while the light power of the semiconductor laser diode increases when temperature decreases. This fluctuation of the light power of the laser diode can cause some errors in reading out information from the disc. Accordingly, the optical disc system includes an automatic laser power control (ALPC) circuit to control the light power of the laser diode to be constant.
FIG. 1 shows a conventional automatic laser power control circuit. A laser diode package 11 includes a laser diode 111 and a monitor diode 112 for monitoring a light power of the laser diode 111. The monitor diode 112 outputs a monitor current I proportional to quantity of the light output from the laser diode 111. A voltage determined by multiplying the monitor current I and resistance R is applied to an amplifying unit 12 via a terminal 6. The amplifying unit 12 amplifies the supplied voltage via first and second amplifiers 121 and 122 into a predetermined amplification factor. The output voltage of the amplifying unit 12 is applied to a base of a transistor 13 via a terminal 5. The transistor 13 turns on/off according to the voltage applied to the base. A collector of the transistor 13 is connected through a coil L to the laser diode 111.
If the light power of the laser diode 111 increases, the monitor current I passing through the monitor diode 112 increases. As the monitor current I increases, a higher voltage than the predetermined voltage corresponding to the constant light power of the laser diode 111 is applied to the terminal 6. Higher voltage applied through the terminal 6 is amplified by the first and the second amplifiers 121 and 122, and the amplified higher voltage is applied to the base of the transistor 13 via terminal 5. As a result, the transistor 13 turns off and the driving current of the laser diode decreases. Accordingly, the increased light power of the laser diode 111 decreases, and thus becomes the same as the above constant light power.
On the contrary, if the light power of the laser diode 111 decreases, the monitor current I passing through the monitor diode 112 decreases and then a lower voltage than the predetermined voltage corresponding to the constant light power of the laser diode 111 is applied to the terminal 6. The lower voltage is amplified by the first and the second amplifiers 121 and 122. The amplified lower voltage is applied to the base of the transistor 13 via the terminal 5, and the transistor 13 turns on. When the transistor 13 turns on, the current passing through the laser diode 111 increases, and the light power of the laser diode 111 increases reach the constant light power corresponding to the predetermined voltage. As described above, the automatic laser power control circuit controls the light power of the laser diode 111 to be constant.
The circuit shown in FIG. 1 uses the laser diode package in which cathodes of the laser diode 111 and the monitor diode 112 are grounded through a common ground terminal. However, there are some commercially available laser diode packages in which an anode of the monitor diode is grounded. The direction of the monitor current of the anode-grounded-type laser diode package is different from that of the monitor current of the cathode-grounded-type laser diode package. Accordingly, the conventional circuit shown in FIG. 1 cannot be used with the two types of the laser diode packages.